Search

Alzheimer's disease (AD) pathogenesis is correlated with the membrane content of various lipid species, including cholesterol, whose interactions with amyloid precursor protein (APP) have been extensively explored. Amyloid-β peptides triggering AD are products of APP cleavage by secretases, which differ depending on the APP and secretase location relative to ordered or disordered membrane microdomains. We used high-resolution NMR to probe the interactions of the cholesterol analog with APP transmembrane domain in two membrane-mimicking systems resembling ordered or perturbed lipid environments (bicelles/micelles). In bicelles, spin-labeled sterol interacted with the peptide near the amphiphilic juxtamembrane region and N-terminal part of APP transmembrane helix, as described earlier for cholesterol. Upon transition into micellar environment, another interaction site appeared where sterol polar head was buried in the hydrophobic core near the hinge region. In MD simulations, sterol moved between three interaction sites, sliding along the polar groove formed by glycine residues composing the dimerization interfaces and flexible hinge of the APP transmembrane domain. Because the lipid environment modulates interactions, the role of lipids in the AD pathogenesis is defined by the state of the entire lipid subsystem rather than the effects of individual lipid species. Cholesterol can interplay with other lipids (polyunsaturated, gangliosides, etc.), determining the outcome of amyloid-β production cascades.

The accelerator-driven subcritical system (ADS) is a competitive option for next-generation nuclear energy systems. Production cross sections of long-lived residual radioactive nuclides in a proton-nuclide reaction are basic quantities for the calculation of accumulated radioactivity in the use of ADS systems. This work presents the production cross sections of 102m Rh and 108m Ag in a natural lead ( nat Pb) target activated by 400 MeV protons. The nat Pb target was irradiated by a 400 MeV proton beam in NRC "Kurchatov Institute" and measured two decades later by a low background gamma spectrometer in China Jinping Underground Laboratory (CJPL). A spectrum analysis method based on simulated single-isotope spectrum and Bayesian peak fitting was employed to compute the activities and production cross sections of the residual nuclides. The experimentally measured cross-sections for 102m Rh and 108m Ag were 0.72 ± 0.05 mb and 0.76 ± 0.09 mb respectively. These values are approximately twice as high as those predicted by the QGSP_INCLXX_HP model and fall within the range predicted by the INCL4+ABLA and LAQGSM+GEM2 models for mass numbers A=102 and A=108. These findings offer new experimental insights for ADS research and provide a practical benchmark for theoretical models concerning proton-lead interactions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

Human InsR, IGF1R, and IRR receptor tyrosine kinases (RTK) of the insulin receptor subfamily play an important role in signaling pathways for a wide range of physiological processes and are directly associated with many pathologies, including neurodegenerative diseases. The disulfide-linked dimeric structure of these receptors is unique among RTKs. Sharing high sequence and structure homology, the receptors differ dramatically in their localization, expression, and functions. In this work, using high-resolution NMR spectroscopy supported by atomistic computer modeling, conformational variability of the transmembrane domains and their interactions with surrounding lipids were found to differ significantly between representatives of the subfamily. Therefore, we suggest that the heterogeneous and highly dynamic membrane environment should be taken into account in the observed diversity of the structural/dynamic organization and mechanisms of activation of InsR, IGF1R, and IRR receptors. This membrane-mediated control of receptor signaling offers an attractive prospect for the development of new targeted therapies for diseases associated with dysfunction of insulin subfamily receptors.

Alzheimer's disease is the most common type of neurodegenerative disease in the world. Genetic evidence strongly suggests that aberrant generation, aggregation, and/or clearance of neurotoxic amyloid-β peptides ( Aβ ) triggers the disease. Aβ accumulates at the points of contact of neurons in ordered cords and fibrils, forming the so-called senile plaques. Aβ isoforms of different lengths are found in healthy human brains regardless of age and appear to play a role in signaling pathways in the brain and to have neuroprotective properties at low concentrations. In recent years, different substances have been developed targeting Aβ production, aggregation, interaction with other molecules, and clearance, including peptide-based drugs. Aβ is a product of sequential cleavage of the membrane glycoprotein APP (amyloid precursor protein) by β- and γ-secretases. A number of familial mutations causing an early onset of the disease have been identified in the APP, especially in its transmembrane domain. The mutations are reported to influence the production, oligomerization, and conformational behavior of Aβ peptides. This review highlights the results of structural studies of the main proteins involved in Alzheimer's disease pathogenesis and the molecular mechanisms by which perspective therapeutic substances can affect Aβ production and nucleation.

Alzheimer's disease is an age-related pathology associated with accumulation of amyloid-β peptides, products of enzymatic cleavage of amyloid-β precursor protein (APP) by secretases. Several familial mutations causing early onset of the disease have been identified in the APP transmembrane (TM) domain. The mutations influence production of amyloid-β, but the molecular mechanisms of this effect are unclear. The "Australian" (L723P) mutation located in the C-termini of APP TM domain is associated with autosomal-dominant, early onset Alzheimer's disease. Herein, we describe the impact of familial L723P mutation on the structural-dynamic behavior of APP TM domain studied by high-resolution NMR in membrane-mimicking micelles and augmented by molecular dynamics simulations in explicit lipid bilayer. We found L723P mutation to cause local unfolding of the C-terminal turn of the APP TM domain helix and increase its accessibility to water required for cleavage of the protein backbone by γ-secretase in the ε-site, thus switching between alternative ("pathogenic" and "non-pathogenic") cleavage cascades. These findings suggest a straightforward mechanism of the pathogenesis associated with this mutation, and are of generic import for understanding the molecular-level events associated with APP sequential proteolysis resulting in accumulation of the pathogenic forms of amyloid-β. Moreover, age-related onset of Alzheimer's disease can be explained by a similar mechanism, where the effect of mutation is emulated by the impact of local environmental factors, such as oxidative stress and/or membrane lipid composition. Knowledge of the mechanisms regulating generation of amyloidogenic peptides of different lengths is essential for development of novel treatment strategies of the Alzheimer's disease.

Bile cast nephropathy is an often overlooked condition of acute renal injury in the setting of high serum bilirubin. While the exact pathophysiology remains unknown, possible mechanisms of renal injury are tubular obstruction from bile casts, direct toxicity from bile acids, and decreased renal perfusion due to hemodynamic changes. We present a patient with hyperbilirubinemia as a result of common bile duct obstruction due to pancreatic adenocarcinoma who developed anuric acute renal injury. Urine analysis showed bile casts that were highly suggestive for bile cast nephropathy. The patient underwent hemodialysis and bile drainage with full restoration of renal function.

Gramicidin A (gA) is a short β-helical peptide known to form conducting channels in lipid membranes because of transbilayer dimerization. The gA conducting dimer, being shorter than the lipid bilayer thickness, deforms the membrane in its vicinity, and the bilayer elastic energy contributes to the gA dimer formation energy. Likewise, membrane incorporation of a gA monomer, which is shorter than the lipid monolayer thickness, creates a void, thereby forcing surrounding lipid molecules to tilt to fill it. The energy of membrane deformation was calculated in the framework of the continuum elasticity theory, taking into account splay, tilt, lateral stretching/compression, Gaussian splay deformations, and external membrane tension. We obtained the interaction energy profiles for two gA monomers located either in the same or in the opposite monolayers. The profiles demonstrated the long-range attraction and short-range repulsion behavior of the monomers resulting from the membrane deformation. Analysis of the profile features revealed conditions under which clusters of gA monomers would not dissipate because of diffusion. The calculated dependence of the dimer formation and decay energy barriers on the membrane elastic properties was in good agreement with the available experimental data and suggested an explanation for a hitherto contentious phenomenon., (Copyright © 2018 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

Background: Prior studies of the human growth hormone receptor (GHR) revealed a distinct role of spatial rearrangements of its dimeric transmembrane domain in signal transduction across membrane. Detailed structural information obtained in the present study allowed elucidating the bases of such rearrangement and provided novel insights into receptor functioning., Methods: We investigated the dimerization of recombinant TMD fragment GHR 254-294 by means of high-resolution NMR in DPC micelles and molecular dynamics in explicit POPC membrane., Results: We resolved two distinct dimeric structures of GHR TMD coexisting in membrane-mimicking micellar environment and providing left- and right-handed helix-helix association via different dimerization motifs. Based on the available mutagenesis data, the conformations correspond to the dormant and active receptor states and are distinguished by cis-trans isomerization of Phe-Pro 266 bond in the transmembrane helix entry. Molecular dynamic relaxations of the structures in lipid bilayer revealed the role of the proline residue in functionally significant rearrangements of the adjacent juxtamembrane region supporting alternation between protein-protein and protein-lipid interactions of this region that can be triggered by ligand binding. Also, the importance of juxtamembrane SS bonding for signal persistency, and somewhat unusual aspects of transmembrane region interaction with water molecules were demonstrated., Conclusions: Two alternative dimeric structures of GHR TMD attributed to dormant and active receptor states interchange via allosteric rearrangements of transmembrane helices and extracellular juxtamembrane regions that support coordination between protein-protein and protein-lipid interactions., General Significance: This study provides a holistic vision of GHR signal transduction across the membrane emphasizing the role of protein-lipid interactions., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Sphingomyelin- and cholesterol- enriched membrane domains, commonly referred to as "rafts" play a crucial role in a large number of intra- and intercellular processes. Recent experiments suggest that not only the volumetric inhomogeneity of lipid distribution in rafts, but also the arrangement of the 1D boundary between the raft and the surrounding membrane is important for the membrane-associated processes. The reason is that the boundary preferentially recruits different peptides, such as HIV (human immunodeficiency virus) fusion peptide. In the present work, we report a theoretical investigation of mechanisms of influence of the raft boundary arrangement upon virus-induced membrane fusion. We theoretically predict that the raft boundary can act as an attractor for viral fusion peptides, which preferentially distribute into the vicinity of the boundary, playing the role of 'line active components' of the membrane ('linactants'). We have calculated the height of the fusion energy barrier and demonstrated that, in the case of fusion between HIV membrane and the target cell, presence of the raft boundary in the vicinity of the fusion site facilitates fusion. The results we obtained can be further generalized to be applicable to other enveloped viruses.

Membrane fusion mediates multiple vital processes in cell life. Specialized proteins mediate the fusion process, and a substantial part of their energy is used for topological rearrangement of the membrane lipid matrix. Therefore, the elastic parameters of lipid bilayers are of crucial importance for fusion processes and for determination of the energy barriers that have to be crossed for the process to take place. In the case of fusion of enveloped viruses (e.g., influenza) with endosomal membrane, the interacting membranes are in an acidic environment, which can affect the membrane's mechanical properties. This factor is often neglected in the analysis of virus-induced membrane fusion. In the present work, we demonstrate that even for membranes composed of zwitterionic lipids, changes of the environmental pH in the physiologically relevant range of 4.0 to 7.5 can affect the rate of the membrane fusion notably. Using a continual model, we demonstrated that the key factor defining the height of the energy barrier is the spontaneous curvature of the lipid monolayer. Changes of this parameter are likely to be caused by rearrangements of the polar part of lipid molecules in response to changes of the pH of the aqueous solution bathing the membrane., Competing Interests: The authors declare no conflict of interest.

Fusion of cellular membranes during normal biological processes, including proliferation, or synaptic transmission, is mediated and controlled by sophisticated protein machinery ensuring the preservation of the vital barrier function of the membrane throughout the process. Fusion of virus particles with host cell membranes is more sparingly arranged and often mediated by a single fusion protein, and the virus can afford to be less discriminative towards the possible different outcomes of fusion attempts. Formation of leaky intermediates was recently observed in some fusion processes, and an alternative trajectory of the process involving formation of π-shaped structures was suggested. In this study, we apply the methods of elasticity theory and Lagrangian formalism augmented by phenomenological and molecular geometry constraints and boundary conditions to investigate the traits of this trajectory and the drivers behind the choice of one of the possible scenarios depending on the properties of the system. The alternative pathway proved to be a dead end, and, depending on the parameters of the participating membranes and fusion proteins, the system can either reversibly enter the corresponding "leaky" configuration or be trapped in it. A parametric study in the biologically relevant range of variables emphasized the fusion protein properties crucial for the choice of the fusion scenario., Competing Interests: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Lipid membranes are extremely stable envelopes allowing cells to survive in various environments and to maintain desired internal composition. Membrane permeation through formation of transversal pores requires substantial external stress. Practically, pores are usually formed by application of lateral tension or transmembrane voltage. Using the same approach as was used for obtaining continuous trajectory of pore formation in the stress-less membrane in the previous article, we now consider the process of pore formation under the external stress. The waiting time to pore formation proved a non-monotonous function of the lateral tension, dropping from infinity at zero tension to a minimum at the tension of several millinewtons per meter. Transmembrane voltage, on the contrary, caused the waiting time to decrease monotonously. Analysis of pore formation trajectories for several lipid species with different spontaneous curvatures and elastic moduli under various external conditions provided instrumental insights into the mechanisms underlying some experimentally observed phenomena.

Lipid membranes serve as effective barriers allowing cells to maintain internal composition differing from that of extracellular medium. Membrane permeation, both natural and artificial, can take place via appearance of transversal pores. The rearrangements of lipids leading to pore formation in the intact membrane are not yet understood in details. We applied continuum elasticity theory to obtain continuous trajectory of pore formation and closure, and analyzed molecular dynamics trajectories of pre-formed pore reseal. We hypothesized that a transversal pore is preceded by a hydrophobic defect: intermediate structure spanning through the membrane, the side walls of which are partially aligned by lipid tails. This prediction was confirmed by our molecular dynamics simulations. Conversion of the hydrophobic defect into the hydrophilic pore required surmounting some energy barrier. A metastable state was found for the hydrophilic pore at the radius of a few nanometers. The dependence of the energy on radius was approximately quadratic for hydrophobic defect and small hydrophilic pore, while for large radii it depended on the radius linearly. The pore energy related to its perimeter, line tension, thus depends of the pore radius. Calculated values of the line tension for large pores were in quantitative agreement with available experimental data.

Among membrane receptors, the single-span receptor protein kinases occupy a broad but specific functional niche determined by distinctive features of the underlying transmembrane signaling mechanisms that are briefly overviewed on the basis of some of the most representative examples, followed by a more detailed discussion of several hierarchical levels of organization and interactions involved. All these levels, including single-molecule interactions (e.g., dimerization, liganding, chemical modifications), local processes (e.g. lipid membrane perturbations, cytoskeletal interactions), and larger scale phenomena (e.g., effects of membrane surface shape or electrochemical potential gradients) appear to be closely integrated to achieve the observed diversity of the receptor functioning. Different species of receptor protein kinases meet their specific functional demands through different structural features defining their responses to stimulation, but certain common patterns exist. Signaling by receptor protein kinases is typically associated with the receptor dimerization and clustering, ligand-induced rearrangements of receptor domains through allosteric conformational transitions with involvement of lipids, release of the sequestered lipids, restriction of receptor diffusion, cytoskeleton and membrane shape remodeling. Understanding of complexity and continuity of the signaling processes can help identifying currently neglected opportunities for influencing the receptor signaling with potential therapeutic implications. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Interaction between transmembrane helices often determines biological activity of membrane proteins. Bitopic proteins, a broad subclass of membrane proteins, form dimers containing two membrane-spanning helices. Some aspects of their structure-function relationship cannot be fully understood without considering the protein-lipid interaction, which can determine the protein conformational ensemble. Experimental and computer modeling data concerning transmembrane parts of bitopic proteins are reviewed in the present paper. They highlight the importance of lipid-protein interactions and resolve certain paradoxes in the behavior of such proteins. Besides, some properties of membrane organization provided a clue to understanding of allosteric interactions between distant parts of proteins. Interactions of these kinds appear to underlie a signaling mechanism, which could be widely employed in the functioning of many membrane proteins. Treatment of membrane proteins as parts of integrated fine-tuned proteolipid system promises new insights into biological function mechanisms and approaches to drug design. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider., (Copyright © 2016 Elsevier B.V. All rights reserved.)

The epidermal growth factor receptor (EGFR) family is an important class of receptor tyrosine kinases, mediating a variety of cellular responses in normal biological processes and in pathological states of multicellular organisms. Different modes of dimerization of the human EGFR transmembrane domain (TMD) in different membrane mimetics recently prompted us to propose a novel signal transduction mechanism based on protein-lipid interaction. However, the experimental evidence for it was originally obtained with slightly different TMD fragments used in the two different mimetics, compromising the validity of the comparison. To eliminate ambiguity, we determined the nuclear magnetic resonance (NMR) structure of the bicelle-incorporated dimer of the EGFR TMD fragment identical to the one previously used in micelles. The NMR results augmented by molecular dynamics simulations confirm the mutual influence of the TMD and lipid environment, as is required for the proposed lipid-mediated activation mechanism. They also reveal the possible functional relevance of a subtle interplay between the concurrent processes in the lipid and protein during signal transduction.

The human epidermal growth factor receptor (EGFR) of HER/ErbB receptor tyrosine kinase family mediates a broad spectrum of cellular responses transducing biochemical signals via lateral dimerization in plasma membrane, while inactive receptors can exist in both monomeric and dimeric forms. Recently, the dimeric conformation of the helical single-span transmembrane domains of HER/ErbB employing the relatively polar N-terminal motifs in a fashion permitting proper kinase activation was experimentally determined. Here we describe the EGFR transmembrane domain dimerization via an alternative weakly polar C-terminal motif A(661)xxxG(665) presumably corresponding to the inactive receptor state. During association, the EGFR transmembrane helices undergo a structural adjustment with adaptation of inter-molecular polar and hydrophobic interactions depending upon the surrounding membrane properties that directly affect the transmembrane helix packing. This might imply that signal transduction through membrane and allosteric regulation are inclusively mediated by coupled protein-protein and protein-lipid interactions, elucidating paradoxically loose linkage between ligand binding and kinase activation., (Copyright © 2016 Elsevier B.V. All rights reserved.)

7-Dehydrocholesterol, an immediate metabolic predecessor of cholesterol, can accumulate in tissues due to some metabolic abnormalities, causing an array of symptoms known as Smith-Lemli-Opitz syndrome. Enrichment of cellular membranes with 7-dehydrocholesterol interferes with normal cell-signaling processes, which involve interaction between rafts and formation of the so-called signaling platforms. In model membranes, cholesterol-based ordered domains usually merge upon contact. According to our experimental data, ordered domains in the model systems where cholesterol is substituted for 7-dehydrocholesterol never merge on the time scale of the experiment, but clusterize into necklace-like aggregates. We attribute such different dynamical behavior to altered properties of the domain boundary. In the framework of thickness mismatch model, we analyzed changes of interaction energy profiles of two approaching domains caused by substitution of cholesterol by 7-dehydrocholesterol. The energy barrier for domain merger is shown to increase notably, with simultaneous appearance of another distinct local energy minimum. Such energy profile is in perfect qualitative agreement with the experimental observations. The observed change of domain dynamics can impair proper interaction between cellular rafts underlying pathologies associated with deviations in cholesterol metabolism.

Background and Purpose: Neoadjuvant chemoradiotherapy (nCRT) improves survival in esophageal cancer (EC) patients, but the response to treatment is heterogeneous and little is known regarding prognostic and predictive markers in these patients. CD44, SOX2 and SHH have been implicated in resistance to CRT, possibly through an association with a cancer stem cell phenotype., Material and Methods: 101 EC patients treated with nCRT and surgery were included. Sufficient pre-treatment biopsy material was present in 71 patients, of which 53 patients were non-complete responders on nCRT (nCR). Protein expression was examined using immunohistochemistry (IHC). Prognostic factors were determined using Cox regression analysis for disease free survival (DFS) and cause specific survival (CSS) in the complete cohort, the pre-treatment biopsies group and post-treatment nCR group., Results: Low CD44 expression in the nCR group was an independent prognostic factor for both DFS and CSS (DFS HR 2.81, p=0.002 and CSS HR 3.48, p=0.002). Absent SOX2 expression in pretreatment biopsies was related to systemic recurrence (p=0.029) while low SHH in pretreatment biopsies was an independent prognostic factor for a poor DFS (HR 2.27, p=0.036). No relation between marker expression and response to nCRT was observed., Conclusions: Low expression of CD44 and SHH are associated with a poor survival outcome in EC patients treated with nCRT., (Copyright © 2015 Elsevier Ireland Ltd. All rights reserved.)

Background: It has been suggested that markers associated with cancer stem cells (CSC) may play a role in esophageal cancer. Our aim was to investigate the expression pattern of proposed CSC markers ALDH1, Axin2, BMI1, CD44, and SOX2 in esophageal adenocarcinoma (EAC) and to relate their expression to survival., Methods: In this study we included 94 EAC patients and examined the expression of the above-mentioned markers by using immunohistochemistry on tissue microarrays. Expression was scored as positive or negative or categorized as low or high in terms of an immunoreactivity score (IRS). Expression rates were related to clinicopathologic characteristics and overall and disease-free survival (DFS)., Results: In a multivariate analysis, negative expression of CD44 and of SOX2 were both significant prognostic factors for DFS [hazard ratio (HR), 1.73; 95 % confidence interval (CI), 1.00-2.96; P = 0.046 and HR, 2.06; 95 % CI 1.14-3.70 P = 0.016). When CD44 and SOX2 expression were analyzed together, negative SOX2 expression was an independent prognostic factor for DFS (HR, 1.91; 95 % CI 1.05-3.46; P = 0.034). Low IRS scores for ALDH1 or Axin2 were associated with a reduced median survival (12.8 vs. 28.7 and 12.1 vs. 25.5 months, respectively). However, these markers and BMI1 were not prognostic factors for survival., Conclusions: Loss of CD44 expression and loss of SOX2 expression are prognostic factors of poor survival in EAC patients. This suggests a role of these proteins in EAC that requires further investigation.

The interaction between transmembrane helices is of great interest because it directly determines biological activity of a membrane protein. Either destroying or enhancing such interactions can result in many diseases related to dysfunction of different tissues in human body. One much studied form of membrane proteins known as bitopic protein is a dimer containing two membrane-spanning helices associating laterally. Establishing structure-function relationship as well as rational design of new types of drugs targeting membrane proteins requires precise structural information about this class of objects. At present time, to investigate spatial structure and internal dynamics of such transmembrane helical dimers, several strategies were developed based mainly on a combination of NMR spectroscopy, optical spectroscopy, protein engineering and molecular modeling. These approaches were successfully applied to homo- and heterodimeric transmembrane fragments of several bitopic proteins, which play important roles in normal and in pathological conditions of human organism.

In any organism, very precisely adjusted interaction and exchange of information between cellsis continuously required. These cooperative interactions involve numerous cytokines, acting throughcorresponding sets of cell-surface receptors. The transforming growth factor β (TGF-β)superfamily includes a variety of structurally related multifunctional cytokines that play criticalroles in maintaining cellular homeostasis and controlling cell fate. Response of a cell to a specificsignal it receives should depend upon the current state of the environment, including concentrationsof biologically relevant ions. One of the most biologically active ions, calcium, acts upon a specificcalcium signaling system that operates over a wide temporal range and regulates many cellularprocesses in continuous "cross-talk" with the TGF-β signaling system. In additionto that, the structural and dynamical properties of TGF-β molecules, along with detected directinteraction of them with the biologically relevant cations suggest another level of fine regulationof TGF-β activity. The fact that both mono- and divalent cations bind in the same low-affinitysites implies that some competition of cations for interaction with TGF-β can also occur in vivo,contributing to the diversity of TGF-β biological functions.

BNip3 is a prominent representative of apoptotic Bcl-2 proteins with rather unique properties initiating an atypical programmed cell death pathway resembling both necrosis and apoptosis. Many Bcl-2 family proteins modulate the permeability state of the outer mitochondrial membrane by forming homo- and hetero-oligomers. The structure and dynamics of the homodimeric transmembrane domain of BNip3 were investigated with the aid of solution NMR in lipid bicelles and molecular dynamics energy relaxation in an explicit lipid bilayer. The right-handed parallel helix-helix structure of the domain with a hydrogen bond-rich His-Ser node in the middle of the membrane, accessibility of the node for water, and continuous hydrophilic track across the membrane suggest that the domain can provide an ion-conducting pathway through the membrane. Incorporation of the BNip3 transmembrane domain into an artificial lipid bilayer resulted in pH-dependent conductivity increase. A possible biological implication of the findings in relation to triggering necrosis-like cell death by BNip3 is discussed.

In 1-adamantyl-2,8,9-trioxa-5-aza-1-germabicyclo[3.3.3]undecane or 1-adamantylgermatrane, [Ge(C(10)H(15))(C(6)H(12)NO(3))], (I), and (2,8,9-trioxa-5-aza-1-germabicyclo[3.3.3]undecan-1-yl)methyl N-cyclohexylcarbamate or [(germatran-1-yl)methyl] N-cyclohexylcarbamate, [Ge(C(6)H(12)NO(3))(C(8)H(14)NO(2))], (II), the Ge atoms are characterized by trigonal-bypiramidal configurations. The Ge...N distances [2.266 (3) and 2.206 (3) A in (I) and (II), respectively] are among the longest observed in germatranes. The significant distortion of the apical N-Ge-C angle in (II) is caused by crystal packing effects.

Based on the (1)H-(15)N NMR spectroscopy data, the three-dimensional structure and internal dynamic properties of ribosomal protein L7 from Escherichia coli were derived. The structure of L7 dimer in solution can be described as a set of three distinct domains, tumbling rather independently and linked via flexible hinge regions. The dimeric N-terminal domain (residues 1-32) consists of two antiparallel alpha-alpha-hairpins forming a symmetrical four-helical bundle, whereas the two identical C-terminal domains (residues 52-120) adopt a compact alpha/beta-fold. There is an indirect evidence of the existence of transitory helical structures at least in the first part (residues 33-43) of the hinge region. Combining structural data for the ribosomal protein L7/L12 from NMR spectroscopy and x-ray crystallography, it was suggested that its hinge region acts as a molecular switch, initiating "ratchet-like" motions of the L7/L12 stalk with respect to the ribosomal surface in response to elongation factor binding and GTP hydrolysis. This hypothesis allows an explanation of events observed during the translation cycle and provides useful insights into the role of protein L7/L12 in the functioning of the ribosome.

Experimental data on the ion electrogenic transport by Na+,K+-ATPase available in the literature are analyzed. Special attention is paid to the measurements of unsteady-state electric currents initiated by alternating voltage or rapid introduction of the substrate. In the final part, a physical model of the Na+,K+-ATPase functioning is discussed. According to this model, active transport is carried out by opening and closing of the access channels used for the sodium and potassium exchange between solutions on either side of the membrane. The model explains most of the experimental data, although some details (the channel size, rates of individual transport steps) need further refinement.